In heating operation of air conditioners that use refrigerant, defrosting of an outdoor unit must be performed during operation and, at that time, cold air is blown from an indoor unit. To solve this, measures have been implemented in which a blowing temperature is monitored and, when the blowing of cold air is detected, a total air volume is reduced. In the background art, technology has been proposed to estimate a blowing temperature from a pipe temperature of a heat exchanger without using a temperature sensor to measure the blowing temperature (e.g. Patent Literature 1).